The Neanderthal was once the apex predator on Earth and was at least as bright as your dopier kinds of people… He had fire and hard stone tools and was able to bind hard stone spear blades to wooden shafts. What happened to him? How did he become altogether extinct?

Basically, it wasn’t any one thing; a number of things happened to him:

• Predator population dynamics. This was the most major thing that happened to him inasmuch as it made the effects of everything else much worse and much more apt to eventually render him extinct, vastly fewer needing to die out for the species to vanish. We see the same thing in Africa, a few hundred or a few thousand lions at most and how many millions of zebra, buffalo, wildabeast…
• Cosmic disasters. Immanuel Velikovsky wrote a book (Worlds in Collision) about cosmic disaster in eearly historical periods, the thing about Isaiah 14 being a prayer of thanksgiving after Venus settled into a safe orbit etc. but cosmic disasters with major impacts on Earth’s living world go back at least some tens of thousands of years to that original transfer of modern CroMagnon humans to Earth (Ganymede Hypothesis). In one such disaster presumably a few thousand years prior to the events Velikovsky described, Earth lost some forty species of megafauna.
• Projectile weapons. Humans are basically aquatic mammals. The motion of the basic human swim stroke is entirely the same as to throw (a rock, a baseball, a javelin…) overhand or use an atlatl. Monkeys and apes lack the shoulder attachments for any of that and the same had to have been true of hominids. The Neanderthal was limited to thrusting spears while humans were flinging javelins and atlatl spears at him. And then there was the thing about predator/prey population dynamics… Imagine being a lion and having to face 2000 buffalo or zebras with javelins and atlatls. That is why African lions will not mess with the Masai…
• Daylight. The Neanderthal with his huge dark-world eyes had to have had major problems dealing with daylight, even as lemurs, tarsiers, and other leftovers from that age are now limited to operating mainly or entirely at night. That robbed him of some periods of the day and probably robbed him of the ability to funnction on open steppe and limited him to forested areas where people like the Basque called him “Basajaun” or “forest Lord”.

Troy McLachlan's analysis of Rocky planets (Earth, Mars…) aligned with a dwarf star (Saturn), the reason for all the darkness and huge eyes: